The present invention relates to an improved susceptor which more uniformly inhibits the deposition of process gasses on the edge and backside of a substrate, and which may be easily cleaned.
Chemical vapor deposition (CVD) is one of a number of processes used to deposit thin films of material on semiconductor substrates. To process substrates using CVD, a vacuum chamber is provided with a susceptor configured to receive a substrate. In a typical CVD chamber, the substrate is placed into and removed from the chamber by a robot blade and is supported by the susceptor during processing. A precursor gas is charged into the vacuum chamber through a gas manifold plate situated above the substrate, where the substrate is heated to process temperatures, generally in the range of about 250xc2x0 to 650xc2x0 C. The precursor gas reacts on the heated substrate surface to deposit a thin layer thereon and to form volatile byproduct gases, which are pumped away through the chamber exhaust system.
A primary goal of substrate processing is to obtain the largest useful surface area, and as a result the greatest number of chips, possible from each substrate. This is highlighted by the recent demands from semiconductor chip manufacturers for zero edge exclusion on the substrates processed, i.e., that no portion of the substrate surface, including the edge of the wafer, be wasted. Some important factors to consider include processing variables that effect the uniformity and thickness of the layer deposited on the substrate, and contaminants that can attach to the substrate and render all or a portion of the substrate useless. Both of these factors should be controlled to maximize the useful surface area for each substrate processed.
One cause of particle contamination in the chamber is deposition of material at the edge or on the backside of the substrate. Substrate edges are typically beveled, making deposition difficult to control over these surfaces. Thus deposition at substrate edges can be nonuniform. Additionally where metal is deposited, it tends to adhere differently to dielectrics than it does to silicon. If a wafer""s dielectric layer does not extend to the bevel, metal may be deposited on a silicon bevel, and cause flaking. This may lead to deposited layers that do not adhere properly to the substrate edge and eventually chip or flake, generating unwanted particles in the chamber.
Additionally, chemical mechanical polishing is often used to smooth the surface of a substrate coated with tungsten or other metals. The act of polishing causes any deposits on the edge and backside surfaces to flake and generate unwanted particles. A number of approaches have been employed to control the deposition of process gasses on the edge of the substrate during processing. One approach employs a shadow ring which essentially masks a portion of the perimeter of the substrate from the process gasses, reducing the overall useful surface area of the substrate. In light of the current demand for zero edge exclusion, this method is becoming less preferred.
Another approach employs a purge ring near the edge of the substrate for delivering a purge gas along the substrate""s edge to thereby prevent edge deposition. The purge gas limits or prevents the deposition gas from reaching the substrate and thus limits or prevents deposition on the wafer""s beveled edge. A third approach uses a shutter ring and a purge ring in combination to form a purge gas chamber having a purge gas inlet and outlet adjacent the substrate""s edge so as to guide the purge gas across the wafer""s edge.
A wafer typically sits just inside (radially) the purge ring, with a gap therebetween. Conventionally purge rings are made of aluminum and are welded to the substrate support in an effort to prevent the ring from deforming during processing. However, during the thermal cycling which occurs within a CVD processing chamber, the aluminum rings nonetheless deform, losing the integrity of their shape and therefore compromise their ability to keep particles from depositing on the substrate""s edge. This can change the size of the gap, leading to non-uniformity of deposition across the wafer""s edge. As the aluminum rings expand and contract, material thereon can flake, and create particles which can contaminate the wafer.
Further, in order for the rings to work effectively for shadowing and particularly for purging, they must be frequently cleaned to remove deposition material which can alter the gap or flake off and deposit on the wafer. Such cleaning increases chamber downtime, reduces throughput and results in higher operating costs. Accordingly a need exists for an improved susceptor which can reliably prevent edge deposition, and which can be easily cleaned.
The present invention overcomes the problems of the prior art by providing a substrate support having a removable edge ring, which is made of a material having a lower coefficient of thermal expansion (CTE), than that of the substrate support. The edge ring and the substrate support are configured for pin and slot coupling. Specifically, either the edge ring, or the substrate support include a plurality of pins, and the other of the edge ring or the substrate support include a plurality of hollow regions or slots in which the pins may be inserted. The slots are at least as wide as a corresponding one of the plurality of pins and extend in the direction in which the substrate support expands and contracts during thermal cycling. Each of the slots extends a length which is sufficient to compensate for the difference between the CTE of the substrate support and the CTE of the edge ring, over the range of process temperatures to which the apparatus is exposed. Preferably the susceptor is made of aluminum, and the edge ring is made of ceramic.
The susceptor""s performance may be further improved by surrounding each of the plurality of pins with a thermally insulating pad, and by ensuring that each slot has a depth which is greater than the length of a corresponding one of the plurality of pins, such that use of the thermally insulating pads thermally insulates both the pins and the edge ring from the substrate support.
The edge ring may be either of the shadow ring and/or purge ring type. The substrate support may include a purge ring and/or a purge gas channel. Three pins which are radially spaced around a substrate supporting surface, and which interface with three slots which extend radially outwardly with respect to the substrate supporting surface, are presently preferred.
The edge ring of the present invention resist de-formation, due to its relatively low CTE, and due to the pin and slot coupling between the edge ring and substrate support. Further, the pin and slot coupling enables the edge ring to be quickly and easily removed for cleaning, and thus reduces chamber downtime.
Other objects, features and advantages of the present invention will become more fully apparent from the following detailed description of the preferred embodiments, the appended claims and the accompanying drawings.